Correct slo middleware position swift proxy-server pipeline

The recommended position for SLO middleware has changed
in Swift (see change ID I7827dd2d9dfbb3c6424773fb2891355d47e372ba).
This change updates the swift proxy-server pipeline to move SLO
middleware to the correct position.

Related-Bug: #1315133
Change-Id: If52935ec7986adec67bb190a4509c8a8c35056c9
Merge "Correct slo middleware position swift proxy-server pipeline"